People are starting to expect these niceties! It's also easier to drop
the config when you know it's there than add it when you don't know it
exists.

Some templates like OTP apps are usable within releases, so the text
does remain a bit vague, but hopefully less than before.

- The OTP app template now adds the supevisor
- The default apps_dir value does not contain the trailing '/' that the
  template index already added.

The first variable can be unkeyed and the provider will sub in the
variable name 'name'. Additionally, the built-in templates and
documentation have been updated so that 'appid' is now 'name' and the
alternative commands are shown.

This totally breaks compatibility with rebar2, and maybe it shouldn't
have.